PTPA and Alzheimer disease: Ginsenoside Rd pretreatment can inhibit tau protein phosphorylation by maintaining a balance of GSK-3β, cyclin-dependent kinase 5 (CDK5/P25), and protein phosphatase 2A (PP-2A) (Li et al., 2013) to inhibit tau phosphorylation of tau protein at Ser199/202, Ser396, or Ser404 in okadaic acid-induced rats, APP transgenic mice, and cortical neurons to increase PP-2A activity for protection against AD (Li et al., 2011a; Li et al., 2021), respectively.